The embodiments disclosed herein relate generally to the field of power generation and, more particularly, to a system and method for recovering waste heat from a plurality of heat sources having different temperatures for generation of electricity.
Enormous amounts of waste heat are generated by a wide variety of industrial and commercial processes and operations. Example sources of waste heat include exhaust streams and heat from space heating assemblies, steam boilers, engines, and cooling systems. Many fuel-burning engines, besides producing an exhaust stream at high temperature, also give off heat at a lower temperature in lubricating oil, cooling fluid, or compressor intercooler air. Although bottoming cycles can be used to recover additional electrical or shaft power from the hot exhaust gases given off by the engine, they are generally not configured to make efficient use of the available lower-temperature heat sources.
When waste heat is low grade, such as waste heat having a temperature below 300 degrees Celsius, for example, conventional heat recovery systems do not operate with sufficient efficiency to make recovery of energy cost-effective. The net result is that vast quantities of waste heat are simply dumped into the atmosphere, ground, or water.
One method to generate electricity from waste heat involves single cycle systems or two-cycle systems that are used in heat recovery applications with waste heat sources of different temperature levels. Single-cycle configurations collect heat from the different waste heat locations in a serial arrangement of heat exchange units with an intermediate heating fluid. This “all-in-one” configuration lowers the maximum resulting fluid vapor temperature because the available heat from the various temperature level heat sources is mixed. An undesirable result of this configuration is a lowered Carnot efficiency. In two-cycle configurations, the hot heat source heats a high-boiling point liquid in a top loop, and the cold heat source heats a low-boiling point liquid in a separate bottom loop. Although, the two-cycle system generally achieves a better performance than a single cycle, components in the two-cycle system are more complex and require more components. As a result, the overall cost of the two-cycle system is significantly higher.
In another conventional system provided to generate electricity from waste heat, a cascaded organic rankine cycle system for utilization of waste heat includes a pair of organic rankine cycle systems, including two working fluids. The cycles are combined, and the respective organic working fluids are chosen such that the organic working fluid of the first organic rankine cycle is condensed at a condensation temperature that is above the boiling point of the organic working fluid of the second organic cycle. A single common heat exchange unit is used for both the condenser of the first organic rankine cycle system and the evaporator of the second organic rankine cycle. A cascaded organic rankine cycle system converts surplus heat into electricity within certain temperature ranges but does not recover waste heat over a wide temperature range, due to an upper temperature limit on the organic fluid of about 250° C. imposed by tendency to degrade rapidly at higher temperatures.
It would be desirable to have a system that effectively recovers waste heat over a wide temperature range from multiple heat sources utilizing a single working fluid.